1. Field of the Invention
The present invention relates to an optical head apparatus for obtaining a spherical aberration error signal to control a spherical aberration correcting means used for recording or reproducing information with respect to an information storage medium such as an optical disk or an optical card, an information recording and reproducing apparatus for recording and reproducing information with respect to an information storage medium, a method for detecting aberration, and a method for adjusting an optical head apparatus.
2. Description of the Prior Art
FIG. 28 shows a configuration of an optical head apparatus of a conventional information recording and reproducing apparatus. Light emitted from a semiconductor laser 101 as a light source is converted by a collimator lens 102 into a parallel beam of light and passes through a beam splitter 103. The beam passing through the beam splitter 103 is converged by an objective lens 105 on an optical disk 107 as an information storage medium. The converged beam is reflected and diffracted by a track on the optical disk 107. The reflected and diffracted beam passes through the objective lens 105 again, is reflected by the beam splitter 103 and is converged by a detecting lens 109. The converged beam is provided with astigmatic aberration in the 45-degree direction with respect to a track by a cylindrical lens 110 and enters a photodetector 901.
FIG. 29 is a front view of the photodetector 901 shown in FIG. 28. The photodetector 901 has detection regions 911 to 914, i.e., an upper left region, an upper right region, a lower right region and a lower left region. The beam 951 provided with astigmatic aberration is adjusted to be located in the center of the detection regions. A focus error signal can be obtained by subtracting a sum signal of signals from two detection regions located diagonally, from a sum signal of signals from two other detection regions located diagonally. Furthermore, a tracking error signal can be obtained by comparing the phase of the sum signal of signals from two detection regions located diagonally with the phase of the sum signal of signals from two other detection regions located diagonally.
In FIG. 28, an actuator 106 moves the objective lens 105 in the direction perpendicular to the track and in the focus direction. The movement in the direction perpendicular to the track is carried out by a tracking control system on the basis of the tracking error signal. The movement in the focus direction is carried out by the focus control system on the basis of the focus error signals. Herein, it is assumed that for the focus error signal generating means, the astigmatic aberration method is used and for the tracking error signal generating means, the phase differential method is used.
In order to increase the density of information recorded on the optical disk 107 as an information storage medium, there has been proposed a system in which a numerical aperture NA of the converging optical system is increased and the wavelength is shortened. However, the increase in the numerical aperture NA leads to the increase in spherical aberration generated due to the thickness error of a substrate as a protective layer of the optical disk 107. As a means for correcting this, a method for generating a spherical wave with a combination of lenses, thereby generating spherical aberration by an objective lens; and a method for generating an opposite spherical aberration with a liquid crystal are known. However, there has been no suitable method for detecting spherical aberration.
Therefore, as the density of information recorded on the information storage medium increases, the optically required condition becomes stricter, which may lead to problems in that a beam cannot be sufficiently narrowed on the information storage medium because of spherical aberration, etc. so that information cannot be recorded stably, and information cannot be reproduced stably.